The present invention relates to folding apparatus, and more particularly to cylinder zig-zag folders such as described in U.S. Pat. No. 3,250,528, issued May 10, 1966 and assigned to the assignee of the present invention. Such folders use a pair of driven cylinders which are mounted with their axes of rotation in parallel adjacent relation. The outer cylindrical walls of the cylinders define a nip which receives the web, and gripper and tucker blades in the cylinder walls alternately fold and draw the web from the nip to deposit it in zig-zag fashion in a stack beneath the cylinders on a moving delivery table.
Folding machines such as that described in the above 3,250,528 patent have satisfactorily and accurately folded continuous, cross perforated webs at very high speeds. Further, since the principal motions are rotary, with very little reciprocating action, the design lends itself to operation at very high speed. Limitations are imposed, however, by the paper web itself, since at higher speeds (in excess of 800 ft./minute) it tends to cling to the surfaces of the cylinders. This problem is solved in part by the use of stripping pins, such as the stripping pins 87 shown in the above-noted 3,250,528 patent, which help separate the web from the cylinders. However, at higher speeds the tendency of the web to cling to the cylinders is so great that the web can actually fold back on itself and be covered by a subsequent fold causing a "dropped" fold in the stack. This is caused by two phenomena: (1) An electric charge between paper and cylinder and, (2) The relatively slow out flow of air from the triangular cavity formed by the folding cylinders and the stacked paper.
High speed movies have shown that after a fold is delivered by the gripper, it bounces off the bar which supports the stripping pins, and the folded edge sticks up and clings to the surface of the cylinder. In the prior art, the next tucker blade would then strike the folded edge urging it back down on the stack and preventing the edge from folding back on itself and being held that way by a subsequent fold.
When the blade strikes the folded edge of the form, however, it may damage the cross perforations, stretching them open in the web direction so that the paper does not feed satisfactorily through subsequent processing equipment, such as a line printer. Sometimes when the web folds back on itself, the blade may catch the web and jam it into the stripping pins as the cylinder continues to rotate. When this happens, the folder must be stopped and cleared, and the broken stripping pins must be replaced. Such accidents are therefore costly both in damaged material (tucker blades, pins, and webs) and in lost production time.
These problems are intermittent in nature, happening mostly at high speeds and depending on many variables, such as the size and weight of the web, temperature and humidity of the paper and the tendency of the paper to hold a curled shape as it is unwound from supply rolls. As operating speeds have become greater and greater, however, the problems have become more and more common, requiring improved means for firmly but gently separating the web from the cylinder without damaging the web, and for reliably and affirmatively laying the zig-zag folded web onto the delivery table.